DD started having "sleepovers" with my parents sometime around her 4th birthday. Around this time we moved into a small house and no longer had a guest bed so my parents started staying in a hotel less than a mile away. When they would be in town DD would sleep at the hotel with them, anywhere from 1 to 3 nights. She loved it. Last summer she went back to my parents house with them for 2 weeks, about 500 miles away. This winter she stayed with them between Thanksgiving and Christmas. She just came back from another 2 week stay.
All of the "big sleepovers" as she calls them begin and end with visits. Sometimes My parents come here and take her back, sometimes I go to their house and she stays on. My parents are very involved with my kids even though they are so far away. We talk on the phone almost daily, keep up to date with fun stuff through flickr, and see each other almost once a month.
DD cosleeps with them when she's there and everyone loves it (even though both of my parents swore they would never do that when she was a baby.
) Both of them can't wait for DS 1.5 to be old enough to stay over too.
On the other hand we have lived with in a mile of my IL's for 3 years now, 5 houses away for 1 of those years, and DD might go a whole month without contact.
I don't see her ever sleeping over their house. There just is not that level of interaction or interest.